The field of the invention relates generally to optical add-drop filters (ADF), and more particularly to add-drop filters including at least one active resonator that provides gain.
Optical add-drop filters (ADFs) have been utilized in applications ranging from optical communication (e.g., modulators, multiplexers, and switches) to optical sensing. These devices typically include two waveguides and a filter. For communication applications, it is important that ADFs have high add-drop efficiencies, low crosstalk, filter tenability to select different wavelengths to add and/or drop, and even different bandwidths. In at least some known systems, Bragg gratings, photonic crystal structures, and whispering gallery mode (WGM) ring resonators have been used as filter components in ADFs. With their micro-scale size, ultra-high quality (Q) factor (which translates into narrow bandwidth), and large free spectral range, WGM resonators have emerged as a relatively attractive candidate for ADFs. However, in addition to fabrication challenges, two problems encountered by at least some known ADFs including WGM resonators are the non-unitary add-drop efficiencies and differences in the efficiencies and crosstalks of the add and drop channels.
Accordingly, current technologies may suffer from significant crosstalk and low drop efficiency, resulting in relatively high losses. Furthermore, at least some known ADFs are not optically tunable. Rather, in at least some known ADFs, bandwidth and frequency tuning may be performed by varying a refractive index of a polymer coating of the WGM resonator, or by directly heating the resonator.